Due to their extraordinarily good ratio of stiffness or strength to density, core composites have a broad range of application especially in the field of aircraft construction.
Well-known conventional core composites are generally formed of an upper and a lower cover layer or cover ply, between which is located, for example, a honeycomb-type core structure formed of vertically extending cells with a hexagonal cross section, for increasing the stiffness of the resulting composite sandwich structure.
For example metallic corrosion-protected aluminum foils, or non-metallic materials, such as Nomex®- or Kevlar®/N636-paper for example, are used for forming the core structure. Both the Nomex®-paper as well as the Kevlare/N636-paper are coated with phenolic resin in a submersion process for increasing the mechanical strength thereof.
Honeycomb-type core structures with cells extending perpendicularly to the major plane of the core possesses a relatively high inherent or self-strength and stiffness, even by themselves, i.e. without the cover layers provided thereon to form a composite sandwich structure. It is therefore readily possible to carry out various processing steps on such a core by itself, without problems and without requiring additional support for the core. For example, such a honeycomb-type core may be submerged in a processing liquid, e.g. for impregnating or coating the core with phenolic resin, or such a core may be mechanically processed or machined, e.g. by boring, milling, cutting, grinding, grit-blasting, etc., because the honeycomb-type core is inherently stiff enough and strong enough to be self-supporting while such processes are carried out.
In contrast to the above described core composites having true honeycomb cell configurations extending perpendicularly to the plane of the core, new types of core composites, especially formed of three-dimensional folded or pleated comb structures, comprise an open or drainable structure. Namely, such pleated or folded core structures include fold or pleat valleys that form open channels extending continuously in the plane of the composite structure, i.e. along or parallel to the cover layers from edge-to-edge of the composite structure. Thus, the core channels of such a core composite remain drainable or ventilatable through the edges even after the opposite major surfaces of the core structure have been covered with the cover layers. Thereby, for example, it is possible to guide various types of lines (e.g. electrical lines, hydraulic lines, water lines, air lines, etc.) through the core composite without previously having to cut or machine a passage therethrough while impairing the mechanical properties of the core composite.
In comparison to true honeycomb-type core structures, however, core structures with a folded or pleated structure may have a lower inherent or self-strength and may thus not be self-supporting before being sandwiched between the cover layers. For this reason, such folded or pleated core structures cannot be subjected to further processing, for example involving a submersion of the core structure in a phenolic resin bath or a mechanical processing or machining, without further measures, because the unsupported core structure would sag, bend, collapse, crush, break, tear or the like when subjected to the processing.
Thus, the standard core processing methods and apparatuses according to the state of the art are not usable without further measures for processing open core structures, for example by surface processing, surface coating, milling, boring, cutting, grinding, grit-blasting, etc.